1. Field of the Invention
The present invention relates to a color separating reduced optical system lens for a color scanner, a color facsimile and the like, and more specifically to a color separating a color separating lens composed of 3 lens units composed of four units and four pieces of lenses.
2. Description of the Prior Art
In a system wherein an original surface is subject to slit exposure scanning, and an image of a slit-like portion to be scanned of the original surface is reduced and focussed by means of a lens on a light receiving surface of a solid state image sensor (for example, CCD) to read an original, since the solid state image sensor is low in MTF (modulation transfer function), a high MTF is required for a lens. Further, in focussing a reduced image on the surface of the solid state image sensor, a read bit position of an original surface and a corresponding read bit position of the solid state image sensor are brought into registration with each other, and therefore, it is desired to minimize a bend of an image, i.e., a distortion. Moreover, since a quantity of light reduces from an axis toward a marginal portion of an original in accordance with the COS.sup.4 rule, an almost perfect vignetting factor up to the extreme marginal portion of the original is required in view of the necessity of increasing the quantity of light in the marginal portion of the original as much as possible on the solid state image sensor.
Also, in recent FAX and the like, machineries are intended to be miniaturized and light-weighted, and even in layouting optical systems, miniaturizing and light-weighting have been desired. In view of this, lenses of less length and less outer diameter are required.
On the other hand, when a color original is color-separated into three colors, R, G and B by a color scanner, a color FAX and the like to focus the respective color information on the solid state image sensor by the lens, a chromatic aberration need be sufficiently compensated unlike a conventional monochromatic reduced optical system lens.
In the conventional monochromatic reduced optical system lens, a quality of light has a wave length in the vicinity of 550 nm which is substantially close to a visual sensitivity, and for compensation of chromatic aberration, approximately 550 nm.+-.50 nm has been satisfactory. However, when a color information of a color original is faithfully readout, since color of the color original is present over the entire visible area, it is necessary to compensate for the chromatic aberration of the lens with respect to the quality of light over the entire visible area. In addition, in a system wherein a color separation element is present in a space between the lens and the solid state image sensor to color-separate a color original into R, G and B, if an optical path length within the color separation element is long, when this color separation element is added to a conventional lens, an aberration of the conventional lens is fallen toward a positive side, failing to attain a sufficient focusing performance. Accordingly, in such a color separation system, a focusing lens is necessary which has a sufficient performance on the solid state image sensor even if the optical path length within the color separation element is long.
The conventional color separating reduced optical system lens has been difficult to meet these requirements as noted above.
When a thick-wall parallel plane glass is added between the conventional focusing lens and the solid state image sensor, an image plane is excessively fallen toward a positive side, deteriorating a focusing performance. Therefore, a focussing lens is necessary which is compensated for aberration when the parallel plane glass is added. This poses a problem in that the overall length of the lens is long and the lens becomes large in size.